Traditional personal listening devices such as headphones and earphones utilize one or more drivers as audio reproduction sources. The drivers convert a signal (which is typically electrical) into mechanical vibrations that cause air pressure waves. These waves travel into the listener's ear canal, where they affect the ear drum (tympanic membrane) and activate the mechanical, bioelectric and biochemical systems connected thereto, resulting in the user's perception of an audible sound corresponding to the signal.
Earphones are the final stage of a signal processing pipeline which allows a sound produced at one time and place, to be heard by somebody at a different time or place. The pipeline may include microphones, analog-to-digital (“A/D”) converters, recorders, mixers, digital-to-analog (“D/A”) converters and/or amplifiers. It is often a goal of each processing stage to avoid unintentional modification of the sound—this is described as a “flat” frequency response, indicating that the various frequency components of the signal are mostly unchanged from input to output, so that the output faithfully reproduces the original sound (or, often, the sound mix prepared by a recording or mixing engineer from recordings of the original musicians' instruments).
One situation that affects earphones and headphones disproportionately often (compared to other “last stage” audio transducers such as full-size loudspeakers) is the use of the earphones with different signal sources. For example, a single pair of headphones may be used at various times with a cellular telephone, a portable music player, an instrument amplifier, or the output of a mixing board at a live concert. The different sources commonly have different output impedances, which give rise to an undesirable audio effect: the headphones produce different output sound, even if the same signal is provided to the next-to-last stage (typically an amplifier of some sort).
This is problematic because headphones and earphones are carefully designed to produce a particular sound from a given input—even if they are not tuned to produce a flat response, they are tuned to produce a desired response, but when driven by a different source, they may fail to perform as expected. Circuit and system design techniques that improve the consistency of headphone audio reproduction when driven by different sources may be of significant value in this field.